With the Euro IV standard on exhaust emissions from heavy goods vehicles coming into effect in 2005, devices for pollution control of NO (or nitrogen oxides) had to be put in place.
The system used by most heavy goods vehicle manufacturers for reducing NOx emissions to the required value generally consists in carrying out a selective catalytic reaction with reducing agents such as urea (“Urea SCR” or selective catalytic reduction using ammonia generated in situ in the exhaust gases by decomposition of urea).
In order to do this, it is necessary to equip the vehicles with a tank containing a urea solution and also with a device for metering the amount of urea to be injected into the exhaust line. Given that the aqueous urea solution generally used for this purpose (eutectic 32.5 wt % urea solution) freezes at minus 11° C., it is necessary to provide a heating device to liquefy the solution in order to be able to inject it into the exhaust line in the event of starting in freezing conditions.
For the purpose of increasing the autonomy of the vehicles, and considering the encumbrance/structure of the vehicles, it is sometimes desired to have at least two tanks for storing the urea solution. For the purpose of preventing the problems of freezing, of limiting the costs of the system and also the power consumed, it is moreover known to heat only one of the two tanks. Such a heating is required because the car manufacturers generally require the anti-polluting system to be effective during at least a given amount of km and/or time if possible also during freezing conditions. Such a specification is or will become more and more applied because of the more and more severe environmental standards. Such a specification implies a minimum volume of liquid to be available considering the consumption of said liquid by the system.
Urea systems with two tanks have already been proposed. Thus, U.S. Pat. No. 5,884,475 describes the use of two urea tanks in series: a main (storage) tank and a secondary (low volume) tank (also referred to as an active tank) which is heated and which is used for starting in the case of freezing. A main urea feed pump sucks up the urea solution into the secondary tank. Patent US '475 does not describe how the situations are managed in which the secondary tank cannot be supplied with urea solution from the storage tank because the latter no longer contains enough urea or because the urea contained in the storage tank is frozen and cannot be transferred from one tank to the other. If the secondary tank cannot be supplied with urea solution so as to contain a sufficient quantity of solution to be injected into the exhaust line, there is a risk of the main pump operating when empty (which could damage it) and of the system not being supplied with additive. Besides, in this document, the main storage tank is heated (either because the entire system is heated (embodiment of FIG. 3) or because said tank is provided with a heater for instance consisting in a heat exchanger circulating engine coolant (embodiment of FIG. 4)), which adds an extra cost to the system.
Moreover, processes are also known from the prior art for transferring urea from a storage tank to a secondary tank using a transfer pump that is kept running throughout the process regardless of the operating conditions, in particular when the urea freezes or when the storage tank does not contain urea for supplying the secondary tank. Such processes may damage the transfer pump.